PROJECT SUMMARY Interactions between the thalamus and prefrontal cortex (PFC) are important for cognitive function in animals ranging from rodents to primates. The importance of these long-range networks is highlighted by multiple neuropsychiatric diseases, including schizophrenia and ADHD. However, most of what we know about thalamo-cortical circuits comes from sensory systems, where primary thalamic inputs arrive in layer 4 (L4). In contrast, the mouse PFC is an agranular area that lacks L4, and instead receives higher-order thalamic inputs directly to superficial layers. We recently discovered that the PFC makes reciprocal connections with both the mediodorsal (MD) and ventromedial (VM) thalamus. These thalamic nuclei support distinct behaviors, but the cellular, synaptic and circuit mechanisms for their interactions with PFC are poorly understood. We found MD strongly drives layer 2/3 (L2/3) pyramidal cells, whereas VM inputs contact the dendrites of a subset of L5 pyramidal cells. Interestingly, both inputs also robustly engage inhibitory networks to drive local inhibition mediated by GABAergic interneurons. The goal of this proposal is to assess how thalamic inputs engage different populations of superficial interneurons to mediate inhibition in the PFC. In Specific Aim 1, we use optogenetics and electrophysiology to study how thalamic inputs drive multiple classes of interneurons in superficial layers. Our preliminary data suggests that MD and VM engage complementary populations of interneurons located in different sub-layers. In Specific Aim 2, we then use conditional optogenetics to study how these interneurons contact excitatory and inhibitory cells within and across layers. Our preliminary data indicate that the interneurons contacted by MD and VM participate in distinct inhibitory and disinhibitory circuits across multiple layers. Lastly, in Specific Aim 3, we combine 1-photon optogenetics with 2-photon microscopy to study how specific populations of interneurons mediate the suppression of dendritic Ca2+ spikes. Our preliminary data reveal that a sub-population of superficial interneurons mediates robust feed-forward inhibition in the dendrites. Together, the results from our experiments will answer fundamental questions about the organization of thalamo-cortical circuits and interneurons in the PFC. They will also help identify potential therapeutic targets for the many neuropsychiatric disorders that arise from disrupted circuitry within the PFC.